An "activation complex" of amino acyl tRNA synthetases (amino acid tRNA ligases E.C. 6.1.1) will be purified from lactating rat mammary gland homogenates. The properties of this complex will be studied with respect to its function in the biosynthesis of proteins. An attempt will be made to correlate the activity of our enzyme preparations with the various particles found in our enzyme preparations and electron microscopy. Our studies on the morphology and function of these activation complexes will be extended to other forms of mammary tissue; of mammary tissue of pregnancy and mammary tumor cells in culture. Evidence will be sought for correlations between the morphology and function of the activation complexes, and the physiological or pathological state of the tissue.